LITTLE ROCK, Ark. (Reuters) - A federal judge on Friday blocked a new Arkansas law that would ban most abortions after 12 weeks of pregnancy, one of the most restrictive abortion statutes in the nation.

U.S. District Judge Susan Webber Wright granted an injunction sought by abortion rights groups, saying the ban was "more than likely" unconstitutional and ordering that implementation be delayed until a legal challenge to the law can be tried in court.

The law was to have taken effect in August.

The Center for Reproductive Rights and the American Civil Liberties Union filed suit against the state medical board in April on behalf of two Arkansas doctors who perform abortions.

"Today's decision ensures that the women of Arkansas will remain protected from this blatant unconstitutional assault on their health and fundamental reproductive rights," Nancy Northup, chief executive of the Center for Reproductive Rights, said in a statement.

"Such an extreme ban on abortion would have immediate and devastating consequences for women in Arkansas, especially those who could not afford to travel out of state to access reproductive health care," she said.

Northup said the group is confident the law will ultimately be struck down in court.

The Arkansas Human Heartbeat Protection Act would ban most abortions after about 12 weeks of pregnancy, once a fetal heartbeat can be detected by a standard ultrasound. It allows exemptions in cases where the pregnancy resulted from rape or incest, if the life of the mother is in danger, or if major fetal problems are detected.

Doctors who violate the law risk having their licenses revoked by the state medical board.

The Republican-controlled legislature passed the measure in March. Arkansas Governor Mike Beebe, a Democrat, vetoed the bill, but the legislature overrode the veto.

Abortion foes said on Friday they were still hopeful the law would ultimately be allowed to go ahead.

"We are disappointed that Judge Susan Webber Wright has stopped the enforcement of this very good pro-life law," said Jerry Cox, president of the anti-abortion Family Council group. "However, we are very hopeful that she will issue a ruling that this law is constitutional when this case is fully debated in court."

Eleven states, including Arkansas, already ban abortions for pregnancies of 20 weeks or longer, according to Kate Bernyk, spokesman for The Center for Reproductive Rights.

Until recently, Arkansas's proposed law was considered the most restrictive in the nation. But North Dakota earlier this year passed a law that critics said could ban abortion as early as six weeks of pregnancy.